Combline Filters for VHF and UHF

RF pollution is rampant at good portable locations on mountaintops and other high places – anywhere accessible is populated with cellphone towers, TV and FM broadcast stations, two-way radio and pager transmitters, and even amateur repeaters. Most of these are high power, producing signals strong enough to seriously overload the VHF and UHF transceivers we use for contest operation or microwave liaison. The problem often manifests itself as a very high noise level. In August 2013, we were operating the 10 GHz & Up Contest from the top of Mt. Mansfield in Vermont, right next to the building with most of the TV and FM transmitters. Our two-meter liaison transceiver was suffering from a very high noise level, so we could only hear strong signals – not much good for working DX. Fortunately, N1JEZ had asked me to bring a filter. We put my combline filter in line and eliminated the excess noise. On previous expeditions, we didn’t have a filter and suffered the consequences with noise, birdies, and interference. The advent of broadband MMIC preamps acerbates the problem. Unfiltered, they would be a disaster on a mountaintop like Mt. Mansfield. Even at my QTH, 42 km away from Mt. Mansfield but line-of-sight, the strongest FM broadcast station, at 107.9 MHz, is -17 dBm on an FM turnstile antenna. Amplified by 25 dB or more, this is more power than most receivers can handle, even out of band. Combline Filters in Stripline I was inspired by a QST article by Reed Fisher, W2CQH, from 1968: “Combline V.H.F. Bandpass Filters.” Making one had been on my “to do” list for years, but I finally got around to it in 2010 after other mountaintop noise and interference problems. A combline filter uses parallel transmission line resonators less than a quarter-wave long, loaded by capacitance at the open end. This allows tuning over a range of frequencies by varying the capacitance. Typical electrical length of the resonators is between 30 and 60 electrical degrees; a quarter-wavelength is 90 degrees. The W2CQH version uses three parallel stripline resonators tuned by air trimmer capacitors, with two additional by air trimmer capacitors input and output coupling. I didn’t like the coupling capacitors for two reasons: they add two additional adjustments, making it hard to tune the filter without good swept test equipment, and, more important, the capacitors are hard to find.